Having been on the same realm since I started playing and watching as it gradually went downhill, I can definitely see where you're coming from.
Checked here: http://wow.realmpop.com/us.html
Not sure how up to date that is...my server definitely isn't the worst of the worst but I can now see the difference between my realm and actual populated realms. 180k characters versus 40k characters is quite a large gap.
For that simple reason I could definitely agree with having a cross-realm AH. My small server is 70% horde/30% alliance and when I played alliance there were times that a single person could have a full and complete domination of some aspect of the AH...like jewelcrafting for instance.
People on smaller servers now have less access to mats to farm due to CRZ but the same problem as always with their AH.